The present application claims priority to Japanese Application No. P11-094149 filed Mar. 31, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a solid electrolyte battery, and more particularly to a solid electrolyte battery incorporating a solid electrolyte layer constituted by two or more layers of solid electrolyte layers.
2. Description of the Related Art
In recent years, a multiplicity of portable electronic apparatuses, such as camcoders, portable telephones and portable computers, have appeared. Reduction in the size and weight of the apparatus has been required. Raising of the energy density of the battery serving as a portable power source of the apparatus has been required. Among a variety of batteries, a battery of a type containing light metal, such as lithium, sodium or aluminum, as a negative-electrode active material, receives attention because of its high energy density.
Batteries of a type containing light metal, such as lithium, to serve as the negative-electrode active material and manganese dioxide (MnO2), fluorocarbon [(CF)n] or thionyl chloride (SOCl2) to serve as the positive-electrode active material have widely been used as power sources of electric calculators and clocks and backup power sources of memories. Secondary batteries have widely been used each of which incorporates a negative-electrode active material which is carbon material, such as graphite or low-crystalline carbon, which occludes/discharges lithium ions. Moreover, the foregoing secondary battery incorporates a positive-electrode active material constituted by a composite lithium oxide mainly composed of LixMO2 (where M is one or more types of transition metal and x usually satisfies the relationship that 0.05xe2x89xa6xc3x97xe2x89xa61.10).
Moreover, research and development of solid electrolyte batteries each incorporating polyethylene oxide or polyphosphagen as the material of the electrolyte have energetically been performed. The solid electrolyte battery, which is free of leakage of electrolytic solution, has advantages that, for example, the structure of the battery can be simplified. Hitherto, the battery of the foregoing type has a layer structure expressed by positive-electrode active-material layer (formed by integrating a positive-electrode mix and a collector)/electrolyte layer (a solid electrolyte)/negative-electrode active material layer (formed by integrating a negative-electrode mix and a connector). Since the electrolyte layer enlarges the area of contact between the active-material layer and the electrolyte layer, either of the following methods is generally employed. That is, the electrolyte layer is formed by coating the active-material layer with a non-solidified electrolyte by a doctor blade method which is a representative method. As an alternative to this, the electrolyte layer is formed by causing a porous film or a unwoven cloth to contain an electrolyte.
The foregoing method of forming the solid electrolyte layer, such as the doctor blade method, wherein the active-material layer is coated with the non-solidified electrolytes cannot easily uniform the thickness of the solid electrolyte layer. Thus, the foregoing method suffers from a problem in that the thickness of the solid electrolyte layer is easily dispersed.
If a solid-electrolyte lithium secondary battery incorporates the solid electrolyte layer which has a nonuniform thickness, the mobility of lithium ions in the solid electrolyte layer is dispersed. Since battery reactions are concentrated on a portion in which the mobility of lithium ions is relatively high, that is, the thickness of the solid electrolyte layer is small. As a result, the battery capacity is reduced, causing the lifetime of the solid-electrolyte lithium secondary battery against charge-discharge cycles to be shortened. If a portion in which the solid electrolyte layer is very thin is present, the insulated state is broken down starting with the foregoing portion in a case where pressure is applied to the solid-electrolyte lithium secondary battery. It leads to a fact that the positive-electrode active-material layer and the negative-electrode active material layer are brought into contact with each other. Thus, an internal short-circuit occurs.
In a case where the electrolyte is formed into a film-like shape to uniform the thickness of the solid electrolyte layer, the solid electrolyte of a type having a high electric conductivity has a low glass transition point Tg because of the characteristics thereof. Therefore, the foregoing solid electrolyte is a soft electrolyte, causing internal short circuit to easily take place. Since the soft solid electrolyte cannot easily be formed into the film-like shape, the foregoing solid electrolyte cannot be employed from a viewpoint of practical use.
When the solid electrolyte is formed by causing an electrolyte to be contained in a porous film or an unwoven cloth, the electric conductivity of the solid electrolyte layer deteriorates because the porous film or the unwoven cloth incorporates the small-size pores by a small number. Thus, the effective resistance of the solid electrolyte layer is raised, causing the characteristics of the battery to deteriorate. Since the unwoven cloth has a structure that the amount of fibers per unit area (grammage) is nonuniform, the mobility of lithium ions is easily dispersed. Since battery reactions are concentrated on a portion in which the mobility of lithium ions is relatively high, that is, in a portion in which the grammage is very small, the battery capacity is reduced. Thus, the lifetime of the solid-electrolyte lithium secondary battery against charge-discharge cycles is shortened. If a portion in which the grammage is very small is present, the insulated state is broken down starting with the foregoing portion in a case where pressure is applied to the solid-electrolyte lithium secondary battery. It leads to a fact that the positive-electrode active-material layer and the negative-electrode active material layer are brought into contact with each other. Thus, internal short-circuit occurs. When a film-shape solid electrolyte is employed which is sufficiently hard to maintain a high electric conductivity and to prevent the internal short circuit, the area of contact between the electrode active material and the solid electrolyte layer is reduced. Therefore, the utilization rate of the electrode cannot be raised and the capacity of the battery is reduced. As a result, the lifetime against charge/discharge cycles is shortened and the load resistance deteriorates.
In view of the foregoing, an object of the present invention is to provide a solid electrolyte battery exhibiting a high utilization rate of the electrodes and satisfactory cycle characteristics.
The inventor of the present invention has energetically performed studies to achieve the foregoing object. Thus, lamination of at least two or more types of solid electrolytes which consists of a soft solid electrolyte having a high electric conductivity and a film-shape solid electrolyte which has a high electric conductivity and which is sufficiently hard to prevent internal short circuit has been detected. Thus, a fact has been found that a structure can be formed with which the area of contact between an electrode active-material layer and an electrolyte layer can be enlarged, the thickness of the electrolyte layer can be uniformed, internal short circuit can be prevented and ion conduction is not inhibited.
That is, according to one aspect of the present invention, there is provided a solid electrolyte battery comprising: a positive electrode; a solid electrolyte layer formed on the positive electrode, said solid electrolyte layer having a multi-layer structure with a plurality of layers; and a negative electrode formed on the solid electrolyte layer, wherein a first solid electrolyte layer in the plurality of layers, the first solid electrolyte layer being the closest layer in the plurality of layers to the positive electrode, and the first solid electrode layer including a polymer having a glass transition point of xe2x88x9260xc2x0 C. or lower when measured by using a differential scanning calorimeter and a number average molecular weight of 100,000 or larger, and at least one of the plurality of layers other than the first solid electrolyte layer being formed by crosslinking a polymer solid electrolyte having a functional group that can be crosslinked.
As described above, the solid electrolyte layer of the layers constituting the solid electrolyte layer having the multi-layer structure which is nearest the positive electrode is constituted by a polymer having a glass transition point of xe2x88x9260xc2x0 C. or lower when measurement is performed by using a differential scanning calorimeter and a number average molecular weight of 100,000 or larger. Therefore, the area of contact between the active-material layer and the electrolyte layer can be enlarged. Moreover, at least one of the layers constituting the solid electrolyte layer having the multi-layer structure except for the layer nearest the positive electrode is formed by crosslinking a polymer solid electrolyte having a functional group which can be crosslinked. Therefore, the thickness of the electrolyte layer can be uniformed so that internal short circuit occurring due to external pressure is prevented.